A number of fiber optic sensors being commercially produced or currently under development are based on the sinusoidally modulated fiber optic interferometer. These include fiber gyroscopes, current sensors, flow sensors, and acoustic sensors. An example of a fiber optic interferometric sensor is described in U.S. Pat. No. 5,644,397, titled Fiber Optic Interferometer Circuit and Magnetic Field Sensor, and issued to Blake on Jul. 1, 1997, hereby incorporated by reference. For example, the fiber gyroscope detects rotation as a phase shift induced in the light waves due to the Sagnac effect. In current and magnetic field sensors, the phase shift in the light waves may be induced by the Faraday effect.
A common method for extracting the desired information from the sensor output is to use an "open loop" signal processing technique. Although "closed loop" signal processing techniques are known to yield superior performance over open loop techniques, the open loop techniques have the advantage of requiring only a single frequency modulator rather than the wide bandwidth modulator required in the closed loop technique.
The output from a sinusoidally modulated interferometric sensor, I.sub.out, can be represented by: ##EQU1## where .omega. is the bias modulation frequency, .phi..sub.R is the phase shift to be measured, .phi..sub.m is the modulation depth, and .theta. is a phase delay uncertainty between the modulation drive signal and the output signal. The most widely used method for determining .phi..sub.R is to synchronously detect the first harmonic component of I.sub.out. The resulting signal is I.sub.o J.sub.1 (.phi..sub.m) cos .theta. sin .phi..sub.R. In order to determine .phi..sub.R from this signal, it is necessary to either measure or stabilize the other three variables, I.sub.o, .phi..sub.m, and .theta.. Consequently, open loop signal processing schemes commonly contain four basic circuits for determining these four parameters. Three of these circuits are synchronous demodulators for the measurement of three different harmonic levels in the output signal, and the fourth circuit is a zero-crossing or level-crossing circuit for determining .theta.. Stable high Q filters are needed for each of the synchronous demodulators and ratioing circuits are needed for comparing the various harmonic levels. In practice, a complete demodulation circuit of this type requires well over a hundred individual electronic components and is quite expensive. Therefore, it is desirable to simplify the demodulation circuitry.